Facial expressivity and vagal tone in 5- and 10-month-old infants

The relationship between heart rate variability and infant emotional expressivity was investigated in 34 5- and 10-month-old infants. Infants were videotaped during a mother and stranger approach paradigm. Heart period and a measure of heart rate variability (cardiac vagal tone) were derived from 3 min of EKG recorded prior to the experiment. Infant facial expressions were coded using Izard's AFFEX system. The frequencies of the emotions of interest and joy and the behavior, look-away, were reliably coded. The durations of interest expressions and look-away behaviors were also coded. Heart rate variability was related to expressivity only for the 5-month-olds. Five-month-old infants with greater vagal tone displayed more interest, more joy, and more look-away behaviors toward the stranger. Five-month-olds also exhibited longer durations of interest expressions than 10-month-old infants. The 10-month-old infants, however, looked away longer than the 5-month-old infants.